muzich/vendor/swiftmailer/notes/rfc/rfc2822.txt

Network Working Group                                 P. Resnick, Editor
Request for Comments: 2822                         QUALCOMM Incorporated
Obsoletes: 822                                                April 2001
Category: Standards Track


                        Internet Message Format

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This standard specifies a syntax for text messages that are sent
   between computer users, within the framework of "electronic mail"
   messages.  This standard supersedes the one specified in Request For
   Comments (RFC) 822, "Standard for the Format of ARPA Internet Text
   Messages", updating it to reflect current practice and incorporating
   incremental changes that were specified in other RFCs.

Table of Contents

   1. Introduction ............................................... 3
   1.1. Scope .................................................... 3
   1.2. Notational conventions ................................... 4
   1.2.1. Requirements notation .................................. 4
   1.2.2. Syntactic notation ..................................... 4
   1.3. Structure of this document ............................... 4
   2. Lexical Analysis of Messages ............................... 5
   2.1. General Description ...................................... 5
   2.1.1. Line Length Limits ..................................... 6
   2.2. Header Fields ............................................ 7
   2.2.1. Unstructured Header Field Bodies ....................... 7
   2.2.2. Structured Header Field Bodies ......................... 7
   2.2.3. Long Header Fields ..................................... 7
   2.3. Body ..................................................... 8
   3. Syntax ..................................................... 9
   3.1. Introduction ............................................. 9
   3.2. Lexical Tokens ........................................... 9



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   3.2.1. Primitive Tokens ....................................... 9
   3.2.2. Quoted characters ......................................10
   3.2.3. Folding white space and comments .......................11
   3.2.4. Atom ...................................................12
   3.2.5. Quoted strings .........................................13
   3.2.6. Miscellaneous tokens ...................................13
   3.3. Date and Time Specification ..............................14
   3.4. Address Specification ....................................15
   3.4.1. Addr-spec specification ................................16
   3.5 Overall message syntax ....................................17
   3.6. Field definitions ........................................18
   3.6.1. The origination date field .............................20
   3.6.2. Originator fields ......................................21
   3.6.3. Destination address fields .............................22
   3.6.4. Identification fields ..................................23
   3.6.5. Informational fields ...................................26
   3.6.6. Resent fields ..........................................26
   3.6.7. Trace fields ...........................................28
   3.6.8. Optional fields ........................................29
   4. Obsolete Syntax ............................................29
   4.1. Miscellaneous obsolete tokens ............................30
   4.2. Obsolete folding white space .............................31
   4.3. Obsolete Date and Time ...................................31
   4.4. Obsolete Addressing ......................................33
   4.5. Obsolete header fields ...................................33
   4.5.1. Obsolete origination date field ........................34
   4.5.2. Obsolete originator fields .............................34
   4.5.3. Obsolete destination address fields ....................34
   4.5.4. Obsolete identification fields .........................35
   4.5.5. Obsolete informational fields ..........................35
   4.5.6. Obsolete resent fields .................................35
   4.5.7. Obsolete trace fields ..................................36
   4.5.8. Obsolete optional fields ...............................36
   5. Security Considerations ....................................36
   6. Bibliography ...............................................37
   7. Editor's Address ...........................................38
   8. Acknowledgements ...........................................39
   Appendix A. Example messages ..................................41
   A.1. Addressing examples ......................................41
   A.1.1. A message from one person to another with simple
          addressing .............................................41
   A.1.2. Different types of mailboxes ...........................42
   A.1.3. Group addresses ........................................43
   A.2. Reply messages ...........................................43
   A.3. Resent messages ..........................................44
   A.4. Messages with trace fields ...............................46
   A.5. White space, comments, and other oddities ................47
   A.6. Obsoleted forms ..........................................47



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   A.6.1. Obsolete addressing ....................................48
   A.6.2. Obsolete dates .........................................48
   A.6.3. Obsolete white space and comments ......................48
   Appendix B. Differences from earlier standards ................49
   Appendix C. Notices ...........................................50
   Full Copyright Statement ......................................51

1. Introduction

1.1. Scope

   This standard specifies a syntax for text messages that are sent
   between computer users, within the framework of "electronic mail"
   messages.  This standard supersedes the one specified in Request For
   Comments (RFC) 822, "Standard for the Format of ARPA Internet Text
   Messages" [RFC822], updating it to reflect current practice and
   incorporating incremental changes that were specified in other RFCs
   [STD3].

   This standard specifies a syntax only for text messages.  In
   particular, it makes no provision for the transmission of images,
   audio, or other sorts of structured data in electronic mail messages.
   There are several extensions published, such as the MIME document
   series [RFC2045, RFC2046, RFC2049], which describe mechanisms for the
   transmission of such data through electronic mail, either by
   extending the syntax provided here or by structuring such messages to
   conform to this syntax.  Those mechanisms are outside of the scope of
   this standard.

   In the context of electronic mail, messages are viewed as having an
   envelope and contents.  The envelope contains whatever information is
   needed to accomplish transmission and delivery.  (See [RFC2821] for a
   discussion of the envelope.)  The contents comprise the object to be
   delivered to the recipient.  This standard applies only to the format
   and some of the semantics of message contents.  It contains no
   specification of the information in the envelope.

   However, some message systems may use information from the contents
   to create the envelope.  It is intended that this standard facilitate
   the acquisition of such information by programs.

   This specification is intended as a definition of what message
   content format is to be passed between systems.  Though some message
   systems locally store messages in this format (which eliminates the
   need for translation between formats) and others use formats that
   differ from the one specified in this standard, local storage is
   outside of the scope of this standard.




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   Note: This standard is not intended to dictate the internal formats
   used by sites, the specific message system features that they are
   expected to support, or any of the characteristics of user interface
   programs that create or read messages.  In addition, this standard
   does not specify an encoding of the characters for either transport
   or storage; that is, it does not specify the number of bits used or
   how those bits are specifically transferred over the wire or stored
   on disk.

1.2. Notational conventions

1.2.1. Requirements notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
   NOT", and "MAY" appear capitalized, they are being used to indicate
   particular requirements of this specification.  A discussion of the
   meanings of these terms appears in [RFC2119].

1.2.2. Syntactic notation

   This standard uses the Augmented Backus-Naur Form (ABNF) notation
   specified in [RFC2234] for the formal definitions of the syntax of
   messages.  Characters will be specified either by a decimal value
   (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by
   a case-insensitive literal value enclosed in quotation marks (e.g.,
   "A" for either uppercase or lowercase A).  See [RFC2234] for the full
   description of the notation.

1.3. Structure of this document

   This document is divided into several sections.

   This section, section 1, is a short introduction to the document.

   Section 2 lays out the general description of a message and its
   constituent parts.  This is an overview to help the reader understand
   some of the general principles used in the later portions of this
   document.  Any examples in this section MUST NOT be taken as
   specification of the formal syntax of any part of a message.

   Section 3 specifies formal ABNF rules for the structure of each part
   of a message (the syntax) and describes the relationship between
   those parts and their meaning in the context of a message (the
   semantics).  That is, it describes the actual rules for the structure
   of each part of a message (the syntax) as well as a description of
   the parts and instructions on how they ought to be interpreted (the
   semantics).  This includes analysis of the syntax and semantics of



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   subparts of messages that have specific structure.  The syntax
   included in section 3 represents messages as they MUST be created.
   There are also notes in section 3 to indicate if any of the options
   specified in the syntax SHOULD be used over any of the others.

   Both sections 2 and 3 describe messages that are legal to generate
   for purposes of this standard.

   Section 4 of this document specifies an "obsolete" syntax.  There are
   references in section 3 to these obsolete syntactic elements.  The
   rules of the obsolete syntax are elements that have appeared in
   earlier revisions of this standard or have previously been widely
   used in Internet messages.  As such, these elements MUST be
   interpreted by parsers of messages in order to be conformant to this
   standard.  However, since items in this syntax have been determined
   to be non-interoperable or to cause significant problems for
   recipients of messages, they MUST NOT be generated by creators of
   conformant messages.

   Section 5 details security considerations to take into account when
   implementing this standard.

   Section 6 is a bibliography of references in this document.

   Section 7 contains the editor's address.

   Section 8 contains acknowledgements.

   Appendix A lists examples of different sorts of messages.  These
   examples are not exhaustive of the types of messages that appear on
   the Internet, but give a broad overview of certain syntactic forms.

   Appendix B lists the differences between this standard and earlier
   standards for Internet messages.

   Appendix C has copyright and intellectual property notices.

2. Lexical Analysis of Messages

2.1. General Description

   At the most basic level, a message is a series of characters.  A
   message that is conformant with this standard is comprised of
   characters with values in the range 1 through 127 and interpreted as
   US-ASCII characters [ASCII].  For brevity, this document sometimes
   refers to this range of characters as simply "US-ASCII characters".





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   Note: This standard specifies that messages are made up of characters
   in the US-ASCII range of 1 through 127.  There are other documents,
   specifically the MIME document series [RFC2045, RFC2046, RFC2047,
   RFC2048, RFC2049], that extend this standard to allow for values
   outside of that range.  Discussion of those mechanisms is not within
   the scope of this standard.

   Messages are divided into lines of characters.  A line is a series of
   characters that is delimited with the two characters carriage-return
   and line-feed; that is, the carriage return (CR) character (ASCII
   value 13) followed immediately by the line feed (LF) character (ASCII
   value 10).  (The carriage-return/line-feed pair is usually written in
   this document as "CRLF".)

   A message consists of header fields (collectively called "the header
   of the message") followed, optionally, by a body.  The header is a
   sequence of lines of characters with special syntax as defined in
   this standard. The body is simply a sequence of characters that
   follows the header and is separated from the header by an empty line
   (i.e., a line with nothing preceding the CRLF).

2.1.1. Line Length Limits

   There are two limits that this standard places on the number of
   characters in a line. Each line of characters MUST be no more than
   998 characters, and SHOULD be no more than 78 characters, excluding
   the CRLF.

   The 998 character limit is due to limitations in many implementations
   which send, receive, or store Internet Message Format messages that
   simply cannot handle more than 998 characters on a line. Receiving
   implementations would do well to handle an arbitrarily large number
   of characters in a line for robustness sake. However, there are so
   many implementations which (in compliance with the transport
   requirements of [RFC2821]) do not accept messages containing more
   than 1000 character including the CR and LF per line, it is important
   for implementations not to create such messages.

   The more conservative 78 character recommendation is to accommodate
   the many implementations of user interfaces that display these
   messages which may truncate, or disastrously wrap, the display of
   more than 78 characters per line, in spite of the fact that such
   implementations are non-conformant to the intent of this
   specification (and that of [RFC2821] if they actually cause
   information to be lost). Again, even though this limitation is put on
   messages, it is encumbant upon implementations which display messages





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   to handle an arbitrarily large number of characters in a line
   (certainly at least up to the 998 character limit) for the sake of
   robustness.

2.2. Header Fields

   Header fields are lines composed of a field name, followed by a colon
   (":"), followed by a field body, and terminated by CRLF.  A field
   name MUST be composed of printable US-ASCII characters (i.e.,
   characters that have values between 33 and 126, inclusive), except
   colon.  A field body may be composed of any US-ASCII characters,
   except for CR and LF.  However, a field body may contain CRLF when
   used in header "folding" and  "unfolding" as described in section
   2.2.3.  All field bodies MUST conform to the syntax described in
   sections 3 and 4 of this standard.

2.2.1. Unstructured Header Field Bodies

   Some field bodies in this standard are defined simply as
   "unstructured" (which is specified below as any US-ASCII characters,
   except for CR and LF) with no further restrictions.  These are
   referred to as unstructured field bodies.  Semantically, unstructured
   field bodies are simply to be treated as a single line of characters
   with no further processing (except for header "folding" and
   "unfolding" as described in section 2.2.3).

2.2.2. Structured Header Field Bodies

   Some field bodies in this standard have specific syntactical
   structure more restrictive than the unstructured field bodies
   described above. These are referred to as "structured" field bodies.
   Structured field bodies are sequences of specific lexical tokens as
   described in sections 3 and 4 of this standard.  Many of these tokens
   are allowed (according to their syntax) to be introduced or end with
   comments (as described in section 3.2.3) as well as the space (SP,
   ASCII value 32) and horizontal tab (HTAB, ASCII value 9) characters
   (together known as the white space characters, WSP), and those WSP
   characters are subject to header "folding" and "unfolding" as
   described in section 2.2.3.  Semantic analysis of structured field
   bodies is given along with their syntax.

2.2.3. Long Header Fields

   Each header field is logically a single line of characters comprising
   the field name, the colon, and the field body.  For convenience
   however, and to deal with the 998/78 character limitations per line,
   the field body portion of a header field can be split into a multiple
   line representation; this is called "folding".  The general rule is



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   that wherever this standard allows for folding white space (not
   simply WSP characters), a CRLF may be inserted before any WSP.  For
   example, the header field:

           Subject: This is a test

   can be represented as:

           Subject: This
            is a test

   Note: Though structured field bodies are defined in such a way that
   folding can take place between many of the lexical tokens (and even
   within some of the lexical tokens), folding SHOULD be limited to
   placing the CRLF at higher-level syntactic breaks.  For instance, if
   a field body is defined as comma-separated values, it is recommended
   that folding occur after the comma separating the structured items in
   preference to other places where the field could be folded, even if
   it is allowed elsewhere.

   The process of moving from this folded multiple-line representation
   of a header field to its single line representation is called
   "unfolding". Unfolding is accomplished by simply removing any CRLF
   that is immediately followed by WSP.  Each header field should be
   treated in its unfolded form for further syntactic and semantic
   evaluation.

2.3. Body

   The body of a message is simply lines of US-ASCII characters.  The
   only two limitations on the body are as follows:

   - CR and LF MUST only occur together as CRLF; they MUST NOT appear
     independently in the body.

   - Lines of characters in the body MUST be limited to 998 characters,
     and SHOULD be limited to 78 characters, excluding the CRLF.

   Note: As was stated earlier, there are other standards documents,
   specifically the MIME documents [RFC2045, RFC2046, RFC2048, RFC2049]
   that extend this standard to allow for different sorts of message
   bodies.  Again, these mechanisms are beyond the scope of this
   document.








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3. Syntax

3.1. Introduction

   The syntax as given in this section defines the legal syntax of
   Internet messages.  Messages that are conformant to this standard
   MUST conform to the syntax in this section.  If there are options in
   this section where one option SHOULD be generated, that is indicated
   either in the prose or in a comment next to the syntax.

   For the defined expressions, a short description of the syntax and
   use is given, followed by the syntax in ABNF, followed by a semantic
   analysis.  Primitive tokens that are used but otherwise unspecified
   come from [RFC2234].

   In some of the definitions, there will be nonterminals whose names
   start with "obs-".  These "obs-" elements refer to tokens defined in
   the obsolete syntax in section 4.  In all cases, these productions
   are to be ignored for the purposes of generating legal Internet
   messages and MUST NOT be used as part of such a message.  However,
   when interpreting messages, these tokens MUST be honored as part of
   the legal syntax.  In this sense, section 3 defines a grammar for
   generation of messages, with "obs-" elements that are to be ignored,
   while section 4 adds grammar for interpretation of messages.

3.2. Lexical Tokens

   The following rules are used to define an underlying lexical
   analyzer, which feeds tokens to the higher-level parsers.  This
   section defines the tokens used in structured header field bodies.

   Note: Readers of this standard need to pay special attention to how
   these lexical tokens are used in both the lower-level and
   higher-level syntax later in the document.  Particularly, the white
   space tokens and the comment tokens defined in section 3.2.3 get used
   in the lower-level tokens defined here, and those lower-level tokens
   are in turn used as parts of the higher-level tokens defined later.
   Therefore, the white space and comments may be allowed in the
   higher-level tokens even though they may not explicitly appear in a
   particular definition.

3.2.1. Primitive Tokens

   The following are primitive tokens referred to elsewhere in this
   standard, but not otherwise defined in [RFC2234].  Some of them will
   not appear anywhere else in the syntax, but they are convenient to
   refer to in other parts of this document.




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   Note: The "specials" below are just such an example.  Though the
   specials token does not appear anywhere else in this standard, it is
   useful for implementers who use tools that lexically analyze
   messages.  Each of the characters in specials can be used to indicate
   a tokenization point in lexical analysis.

NO-WS-CTL       =       %d1-8 /         ; US-ASCII control characters
                        %d11 /          ;  that do not include the
                        %d12 /          ;  carriage return, line feed,
                        %d14-31 /       ;  and white space characters
                        %d127

text            =       %d1-9 /         ; Characters excluding CR and LF
                        %d11 /
                        %d12 /
                        %d14-127 /
                        obs-text

specials        =       "(" / ")" /     ; Special characters used in
                        "<" / ">" /     ;  other parts of the syntax
                        "[" / "]" /
                        ":" / ";" /
                        "@" / "\" /
                        "," / "." /
                        DQUOTE

   No special semantics are attached to these tokens.  They are simply
   single characters.

3.2.2. Quoted characters

   Some characters are reserved for special interpretation, such as
   delimiting lexical tokens.  To permit use of these characters as
   uninterpreted data, a quoting mechanism is provided.

quoted-pair     =       ("\" text) / obs-qp

   Where any quoted-pair appears, it is to be interpreted as the text
   character alone.  That is to say, the "\" character that appears as
   part of a quoted-pair is semantically "invisible".

   Note: The "\" character may appear in a message where it is not part
   of a quoted-pair.  A "\" character that does not appear in a
   quoted-pair is not semantically invisible.  The only places in this
   standard where quoted-pair currently appears are ccontent, qcontent,
   dcontent, no-fold-quote, and no-fold-literal.





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3.2.3. Folding white space and comments

   White space characters, including white space used in folding
   (described in section 2.2.3), may appear between many elements in
   header field bodies.  Also, strings of characters that are treated as
   comments may be included in structured field bodies as characters
   enclosed in parentheses.  The following defines the folding white
   space (FWS) and comment constructs.

   Strings of characters enclosed in parentheses are considered comments
   so long as they do not appear within a "quoted-string", as defined in
   section 3.2.5.  Comments may nest.

   There are several places in this standard where comments and FWS may
   be freely inserted.  To accommodate that syntax, an additional token
   for "CFWS" is defined for places where comments and/or FWS can occur.
   However, where CFWS occurs in this standard, it MUST NOT be inserted
   in such a way that any line of a folded header field is made up
   entirely of WSP characters and nothing else.

FWS             =       ([*WSP CRLF] 1*WSP) /   ; Folding white space
                        obs-FWS

ctext           =       NO-WS-CTL /     ; Non white space controls

                        %d33-39 /       ; The rest of the US-ASCII
                        %d42-91 /       ;  characters not including "(",
                        %d93-126        ;  ")", or "\"

ccontent        =       ctext / quoted-pair / comment

comment         =       "(" *([FWS] ccontent) [FWS] ")"

CFWS            =       *([FWS] comment) (([FWS] comment) / FWS)

   Throughout this standard, where FWS (the folding white space token)
   appears, it indicates a place where header folding, as discussed in
   section 2.2.3, may take place.  Wherever header folding appears in a
   message (that is, a header field body containing a CRLF followed by
   any WSP), header unfolding (removal of the CRLF) is performed before
   any further lexical analysis is performed on that header field
   according to this standard.  That is to say, any CRLF that appears in
   FWS is semantically "invisible."

   A comment is normally used in a structured field body to provide some
   human readable informational text.  Since a comment is allowed to
   contain FWS, folding is permitted within the comment.  Also note that
   since quoted-pair is allowed in a comment, the parentheses and



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   backslash characters may appear in a comment so long as they appear
   as a quoted-pair.  Semantically, the enclosing parentheses are not
   part of the comment; the comment is what is contained between the two
   parentheses.  As stated earlier, the "\" in any quoted-pair and the
   CRLF in any FWS that appears within the comment are semantically
   "invisible" and therefore not part of the comment either.

   Runs of FWS, comment or CFWS that occur between lexical tokens in a
   structured field header are semantically interpreted as a single
   space character.

3.2.4. Atom

   Several productions in structured header field bodies are simply
   strings of certain basic characters.  Such productions are called
   atoms.

   Some of the structured header field bodies also allow the period
   character (".", ASCII value 46) within runs of atext.  An additional
   "dot-atom" token is defined for those purposes.

atext           =       ALPHA / DIGIT / ; Any character except controls,
                        "!" / "#" /     ;  SP, and specials.
                        "$" / "%" /     ;  Used for atoms
                        "&" / "'" /
                        "*" / "+" /
                        "-" / "/" /
                        "=" / "?" /
                        "^" / "_" /
                        "`" / "{" /
                        "|" / "}" /
                        "~"

atom            =       [CFWS] 1*atext [CFWS]

dot-atom        =       [CFWS] dot-atom-text [CFWS]

dot-atom-text   =       1*atext *("." 1*atext)

   Both atom and dot-atom are interpreted as a single unit, comprised of
   the string of characters that make it up.  Semantically, the optional
   comments and FWS surrounding the rest of the characters are not part
   of the atom; the atom is only the run of atext characters in an atom,
   or the atext and "." characters in a dot-atom.







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3.2.5. Quoted strings

   Strings of characters that include characters other than those
   allowed in atoms may be represented in a quoted string format, where
   the characters are surrounded by quote (DQUOTE, ASCII value 34)
   characters.

qtext           =       NO-WS-CTL /     ; Non white space controls

                        %d33 /          ; The rest of the US-ASCII
                        %d35-91 /       ;  characters not including "\"
                        %d93-126        ;  or the quote character

qcontent        =       qtext / quoted-pair

quoted-string   =       [CFWS]
                        DQUOTE *([FWS] qcontent) [FWS] DQUOTE
                        [CFWS]

   A quoted-string is treated as a unit.  That is, quoted-string is
   identical to atom, semantically.  Since a quoted-string is allowed to
   contain FWS, folding is permitted.  Also note that since quoted-pair
   is allowed in a quoted-string, the quote and backslash characters may
   appear in a quoted-string so long as they appear as a quoted-pair.

   Semantically, neither the optional CFWS outside of the quote
   characters nor the quote characters themselves are part of the
   quoted-string; the quoted-string is what is contained between the two
   quote characters.  As stated earlier, the "\" in any quoted-pair and
   the CRLF in any FWS/CFWS that appears within the quoted-string are
   semantically "invisible" and therefore not part of the quoted-string
   either.

3.2.6. Miscellaneous tokens

   Three additional tokens are defined, word and phrase for combinations
   of atoms and/or quoted-strings, and unstructured for use in
   unstructured header fields and in some places within structured
   header fields.

word            =       atom / quoted-string

phrase          =       1*word / obs-phrase








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utext           =       NO-WS-CTL /     ; Non white space controls
                        %d33-126 /      ; The rest of US-ASCII
                        obs-utext

unstructured    =       *([FWS] utext) [FWS]

3.3. Date and Time Specification

   Date and time occur in several header fields.  This section specifies
   the syntax for a full date and time specification.  Though folding
   white space is permitted throughout the date-time specification, it
   is RECOMMENDED that a single space be used in each place that FWS
   appears (whether it is required or optional); some older
   implementations may not interpret other occurrences of folding white
   space correctly.

date-time       =       [ day-of-week "," ] date FWS time [CFWS]

day-of-week     =       ([FWS] day-name) / obs-day-of-week

day-name        =       "Mon" / "Tue" / "Wed" / "Thu" /
                        "Fri" / "Sat" / "Sun"

date            =       day month year

year            =       4*DIGIT / obs-year

month           =       (FWS month-name FWS) / obs-month

month-name      =       "Jan" / "Feb" / "Mar" / "Apr" /
                        "May" / "Jun" / "Jul" / "Aug" /
                        "Sep" / "Oct" / "Nov" / "Dec"

day             =       ([FWS] 1*2DIGIT) / obs-day

time            =       time-of-day FWS zone

time-of-day     =       hour ":" minute [ ":" second ]

hour            =       2DIGIT / obs-hour

minute          =       2DIGIT / obs-minute

second          =       2DIGIT / obs-second

zone            =       (( "+" / "-" ) 4DIGIT) / obs-zone





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   The day is the numeric day of the month.  The year is any numeric
   year 1900 or later.

   The time-of-day specifies the number of hours, minutes, and
   optionally seconds since midnight of the date indicated.

   The date and time-of-day SHOULD express local time.

   The zone specifies the offset from Coordinated Universal Time (UTC,
   formerly referred to as "Greenwich Mean Time") that the date and
   time-of-day represent.  The "+" or "-" indicates whether the
   time-of-day is ahead of (i.e., east of) or behind (i.e., west of)
   Universal Time.  The first two digits indicate the number of hours
   difference from Universal Time, and the last two digits indicate the
   number of minutes difference from Universal Time.  (Hence, +hhmm
   means +(hh * 60 + mm) minutes, and -hhmm means -(hh * 60 + mm)
   minutes).  The form "+0000" SHOULD be used to indicate a time zone at
   Universal Time.  Though "-0000" also indicates Universal Time, it is
   used to indicate that the time was generated on a system that may be
   in a local time zone other than Universal Time and therefore
   indicates that the date-time contains no information about the local
   time zone.

   A date-time specification MUST be semantically valid.  That is, the
   day-of-the-week (if included) MUST be the day implied by the date,
   the numeric day-of-month MUST be between 1 and the number of days
   allowed for the specified month (in the specified year), the
   time-of-day MUST be in the range 00:00:00 through 23:59:60 (the
   number of seconds allowing for a leap second; see [STD12]), and the
   zone MUST be within the range -9959 through +9959.

3.4. Address Specification

   Addresses occur in several message header fields to indicate senders
   and recipients of messages.  An address may either be an individual
   mailbox, or a group of mailboxes.

address         =       mailbox / group

mailbox         =       name-addr / addr-spec

name-addr       =       [display-name] angle-addr

angle-addr      =       [CFWS] "<" addr-spec ">" [CFWS] / obs-angle-addr

group           =       display-name ":" [mailbox-list / CFWS] ";"
                        [CFWS]




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display-name    =       phrase

mailbox-list    =       (mailbox *("," mailbox)) / obs-mbox-list

address-list    =       (address *("," address)) / obs-addr-list

   A mailbox receives mail.  It is a conceptual entity which does not
   necessarily pertain to file storage.  For example, some sites may
   choose to print mail on a printer and deliver the output to the
   addressee's desk.  Normally, a mailbox is comprised of two parts: (1)
   an optional display name that indicates the name of the recipient
   (which could be a person or a system) that could be displayed to the
   user of a mail application, and (2) an addr-spec address enclosed in
   angle brackets ("<" and ">").  There is also an alternate simple form
   of a mailbox where the addr-spec address appears alone, without the
   recipient's name or the angle brackets.  The Internet addr-spec
   address is described in section 3.4.1.

   Note: Some legacy implementations used the simple form where the
   addr-spec appears without the angle brackets, but included the name
   of the recipient in parentheses as a comment following the addr-spec.
   Since the meaning of the information in a comment is unspecified,
   implementations SHOULD use the full name-addr form of the mailbox,
   instead of the legacy form, to specify the display name associated
   with a mailbox.  Also, because some legacy implementations interpret
   the comment, comments generally SHOULD NOT be used in address fields
   to avoid confusing such implementations.

   When it is desirable to treat several mailboxes as a single unit
   (i.e., in a distribution list), the group construct can be used.  The
   group construct allows the sender to indicate a named group of
   recipients. This is done by giving a display name for the group,
   followed by a colon, followed by a comma separated list of any number
   of mailboxes (including zero and one), and ending with a semicolon.
   Because the list of mailboxes can be empty, using the group construct
   is also a simple way to communicate to recipients that the message
   was sent to one or more named sets of recipients, without actually
   providing the individual mailbox address for each of those
   recipients.

3.4.1. Addr-spec specification

   An addr-spec is a specific Internet identifier that contains a
   locally interpreted string followed by the at-sign character ("@",
   ASCII value 64) followed by an Internet domain.  The locally
   interpreted string is either a quoted-string or a dot-atom.  If the
   string can be represented as a dot-atom (that is, it contains no
   characters other than atext characters or "." surrounded by atext



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   characters), then the dot-atom form SHOULD be used and the
   quoted-string form SHOULD NOT be used. Comments and folding white
   space SHOULD NOT be used around the "@" in the addr-spec.

addr-spec       =       local-part "@" domain

local-part      =       dot-atom / quoted-string / obs-local-part

domain          =       dot-atom / domain-literal / obs-domain

domain-literal  =       [CFWS] "[" *([FWS] dcontent) [FWS] "]" [CFWS]

dcontent        =       dtext / quoted-pair

dtext           =       NO-WS-CTL /     ; Non white space controls

                        %d33-90 /       ; The rest of the US-ASCII
                        %d94-126        ;  characters not including "[",
                                        ;  "]", or "\"

   The domain portion identifies the point to which the mail is
   delivered. In the dot-atom form, this is interpreted as an Internet
   domain name (either a host name or a mail exchanger name) as
   described in [STD3, STD13, STD14].  In the domain-literal form, the
   domain is interpreted as the literal Internet address of the
   particular host.  In both cases, how addressing is used and how
   messages are transported to a particular host is covered in the mail
   transport document [RFC2821].  These mechanisms are outside of the
   scope of this document.

   The local-part portion is a domain dependent string.  In addresses,
   it is simply interpreted on the particular host as a name of a
   particular mailbox.

3.5 Overall message syntax

   A message consists of header fields, optionally followed by a message
   body.  Lines in a message MUST be a maximum of 998 characters
   excluding the CRLF, but it is RECOMMENDED that lines be limited to 78
   characters excluding the CRLF.  (See section 2.1.1 for explanation.)
   In a message body, though all of the characters listed in the text
   rule MAY be used, the use of US-ASCII control characters (values 1
   through 8, 11, 12, and 14 through 31) is discouraged since their
   interpretation by receivers for display is not guaranteed.







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message         =       (fields / obs-fields)
                        [CRLF body]

body            =       *(*998text CRLF) *998text

   The header fields carry most of the semantic information and are
   defined in section 3.6.  The body is simply a series of lines of text
   which are uninterpreted for the purposes of this standard.

3.6. Field definitions

   The header fields of a message are defined here.  All header fields
   have the same general syntactic structure: A field name, followed by
   a colon, followed by the field body.  The specific syntax for each
   header field is defined in the subsequent sections.

   Note: In the ABNF syntax for each field in subsequent sections, each
   field name is followed by the required colon.  However, for brevity
   sometimes the colon is not referred to in the textual description of
   the syntax.  It is, nonetheless, required.

   It is important to note that the header fields are not guaranteed to
   be in a particular order.  They may appear in any order, and they
   have been known to be reordered occasionally when transported over
   the Internet.  However, for the purposes of this standard, header
   fields SHOULD NOT be reordered when a message is transported or
   transformed.  More importantly, the trace header fields and resent
   header fields MUST NOT be reordered, and SHOULD be kept in blocks
   prepended to the message.  See sections 3.6.6 and 3.6.7 for more
   information.

   The only required header fields are the origination date field and
   the originator address field(s).  All other header fields are
   syntactically optional.  More information is contained in the table
   following this definition.

fields          =       *(trace
                          *(resent-date /
                           resent-from /
                           resent-sender /
                           resent-to /
                           resent-cc /
                           resent-bcc /
                           resent-msg-id))
                        *(orig-date /
                        from /
                        sender /
                        reply-to /



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                        to /
                        cc /
                        bcc /
                        message-id /
                        in-reply-to /
                        references /
                        subject /
                        comments /
                        keywords /
                        optional-field)

   The following table indicates limits on the number of times each
   field may occur in a message header as well as any special
   limitations on the use of those fields.  An asterisk next to a value
   in the minimum or maximum column indicates that a special restriction
   appears in the Notes column.

Field           Min number      Max number      Notes

trace           0               unlimited       Block prepended - see
                                                3.6.7

resent-date     0*              unlimited*      One per block, required
                                                if other resent fields
                                                present - see 3.6.6

resent-from     0               unlimited*      One per block - see
                                                3.6.6

resent-sender   0*              unlimited*      One per block, MUST
                                                occur with multi-address
                                                resent-from - see 3.6.6

resent-to       0               unlimited*      One per block - see
                                                3.6.6

resent-cc       0               unlimited*      One per block - see
                                                3.6.6

resent-bcc      0               unlimited*      One per block - see
                                                3.6.6

resent-msg-id   0               unlimited*      One per block - see
                                                3.6.6

orig-date       1               1

from            1               1               See sender and 3.6.2



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sender          0*              1               MUST occur with multi-
                                                address from - see 3.6.2

reply-to        0               1

to              0               1

cc              0               1

bcc             0               1

message-id      0*              1               SHOULD be present - see
                                                3.6.4

in-reply-to     0*              1               SHOULD occur in some
                                                replies - see 3.6.4

references      0*              1               SHOULD occur in some
                                                replies - see 3.6.4

subject         0               1

comments        0               unlimited

keywords        0               unlimited

optional-field  0               unlimited

   The exact interpretation of each field is described in subsequent
   sections.

3.6.1. The origination date field

   The origination date field consists of the field name "Date" followed
   by a date-time specification.

orig-date       =       "Date:" date-time CRLF

   The origination date specifies the date and time at which the creator
   of the message indicated that the message was complete and ready to
   enter the mail delivery system.  For instance, this might be the time
   that a user pushes the "send" or "submit" button in an application
   program.  In any case, it is specifically not intended to convey the
   time that the message is actually transported, but rather the time at
   which the human or other creator of the message has put the message
   into its final form, ready for transport.  (For example, a portable
   computer user who is not connected to a network might queue a message




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   for delivery.  The origination date is intended to contain the date
   and time that the user queued the message, not the time when the user
   connected to the network to send the message.)

3.6.2. Originator fields

   The originator fields of a message consist of the from field, the
   sender field (when applicable), and optionally the reply-to field.
   The from field consists of the field name "From" and a
   comma-separated list of one or more mailbox specifications.  If the
   from field contains more than one mailbox specification in the
   mailbox-list, then the sender field, containing the field name
   "Sender" and a single mailbox specification, MUST appear in the
   message.  In either case, an optional reply-to field MAY also be
   included, which contains the field name "Reply-To" and a
   comma-separated list of one or more addresses.

from            =       "From:" mailbox-list CRLF

sender          =       "Sender:" mailbox CRLF

reply-to        =       "Reply-To:" address-list CRLF

   The originator fields indicate the mailbox(es) of the source of the
   message.  The "From:" field specifies the author(s) of the message,
   that is, the mailbox(es) of the person(s) or system(s) responsible
   for the writing of the message.  The "Sender:" field specifies the
   mailbox of the agent responsible for the actual transmission of the
   message.  For example, if a secretary were to send a message for
   another person, the mailbox of the secretary would appear in the
   "Sender:" field and the mailbox of the actual author would appear in
   the "From:" field.  If the originator of the message can be indicated
   by a single mailbox and the author and transmitter are identical, the
   "Sender:" field SHOULD NOT be used.  Otherwise, both fields SHOULD
   appear.

   The originator fields also provide the information required when
   replying to a message.  When the "Reply-To:" field is present, it
   indicates the mailbox(es) to which the author of the message suggests
   that replies be sent.  In the absence of the "Reply-To:" field,
   replies SHOULD by default be sent to the mailbox(es) specified in the
   "From:" field unless otherwise specified by the person composing the
   reply.

   In all cases, the "From:" field SHOULD NOT contain any mailbox that
   does not belong to the author(s) of the message.  See also section
   3.6.3 for more information on forming the destination addresses for a
   reply.



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3.6.3. Destination address fields

   The destination fields of a message consist of three possible fields,
   each of the same form: The field name, which is either "To", "Cc", or
   "Bcc", followed by a comma-separated list of one or more addresses
   (either mailbox or group syntax).

to              =       "To:" address-list CRLF

cc              =       "Cc:" address-list CRLF

bcc             =       "Bcc:" (address-list / [CFWS]) CRLF

   The destination fields specify the recipients of the message.  Each
   destination field may have one or more addresses, and each of the
   addresses indicate the intended recipients of the message.  The only
   difference between the three fields is how each is used.

   The "To:" field contains the address(es) of the primary recipient(s)
   of the message.

   The "Cc:" field (where the "Cc" means "Carbon Copy" in the sense of
   making a copy on a typewriter using carbon paper) contains the
   addresses of others who are to receive the message, though the
   content of the message may not be directed at them.

   The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains
   addresses of recipients of the message whose addresses are not to be
   revealed to other recipients of the message.  There are three ways in
   which the "Bcc:" field is used.  In the first case, when a message
   containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is
   removed even though all of the recipients (including those specified
   in the "Bcc:" field) are sent a copy of the message.  In the second
   case, recipients specified in the "To:" and "Cc:" lines each are sent
   a copy of the message with the "Bcc:" line removed as above, but the
   recipients on the "Bcc:" line get a separate copy of the message
   containing a "Bcc:" line.  (When there are multiple recipient
   addresses in the "Bcc:" field, some implementations actually send a
   separate copy of the message to each recipient with a "Bcc:"
   containing only the address of that particular recipient.) Finally,
   since a "Bcc:" field may contain no addresses, a "Bcc:" field can be
   sent without any addresses indicating to the recipients that blind
   copies were sent to someone.  Which method to use with "Bcc:" fields
   is implementation dependent, but refer to the "Security
   Considerations" section of this document for a discussion of each.






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   When a message is a reply to another message, the mailboxes of the
   authors of the original message (the mailboxes in the "From:" field)
   or mailboxes specified in the "Reply-To:" field (if it exists) MAY
   appear in the "To:" field of the reply since these would normally be
   the primary recipients of the reply.  If a reply is sent to a message
   that has destination fields, it is often desirable to send a copy of
   the reply to all of the recipients of the message, in addition to the
   author.  When such a reply is formed, addresses in the "To:" and
   "Cc:" fields of the original message MAY appear in the "Cc:" field of
   the reply, since these are normally secondary recipients of the
   reply.  If a "Bcc:" field is present in the original message,
   addresses in that field MAY appear in the "Bcc:" field of the reply,
   but SHOULD NOT appear in the "To:" or "Cc:" fields.

   Note: Some mail applications have automatic reply commands that
   include the destination addresses of the original message in the
   destination addresses of the reply.  How those reply commands behave
   is implementation dependent and is beyond the scope of this document.
   In particular, whether or not to include the original destination
   addresses when the original message had a "Reply-To:" field is not
   addressed here.

3.6.4. Identification fields

   Though optional, every message SHOULD have a "Message-ID:" field.
   Furthermore, reply messages SHOULD have "In-Reply-To:" and
   "References:" fields as appropriate, as described below.

   The "Message-ID:" field contains a single unique message identifier.
   The "References:" and "In-Reply-To:" field each contain one or more
   unique message identifiers, optionally separated by CFWS.

   The message identifier (msg-id) is similar in syntax to an angle-addr
   construct without the internal CFWS.

message-id      =       "Message-ID:" msg-id CRLF

in-reply-to     =       "In-Reply-To:" 1*msg-id CRLF

references      =       "References:" 1*msg-id CRLF

msg-id          =       [CFWS] "<" id-left "@" id-right ">" [CFWS]

id-left         =       dot-atom-text / no-fold-quote / obs-id-left

id-right        =       dot-atom-text / no-fold-literal / obs-id-right

no-fold-quote   =       DQUOTE *(qtext / quoted-pair) DQUOTE



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no-fold-literal =       "[" *(dtext / quoted-pair) "]"

   The "Message-ID:" field provides a unique message identifier that
   refers to a particular version of a particular message.  The
   uniqueness of the message identifier is guaranteed by the host that
   generates it (see below).  This message identifier is intended to be
   machine readable and not necessarily meaningful to humans.  A message
   identifier pertains to exactly one instantiation of a particular
   message; subsequent revisions to the message each receive new message
   identifiers.

   Note: There are many instances when messages are "changed", but those
   changes do not constitute a new instantiation of that message, and
   therefore the message would not get a new message identifier.  For
   example, when messages are introduced into the transport system, they
   are often prepended with additional header fields such as trace
   fields (described in section 3.6.7) and resent fields (described in
   section 3.6.6).  The addition of such header fields does not change
   the identity of the message and therefore the original "Message-ID:"
   field is retained.  In all cases, it is the meaning that the sender
   of the message wishes to convey (i.e., whether this is the same
   message or a different message) that determines whether or not the
   "Message-ID:" field changes, not any particular syntactic difference
   that appears (or does not appear) in the message.

   The "In-Reply-To:" and "References:" fields are used when creating a
   reply to a message.  They hold the message identifier of the original
   message and the message identifiers of other messages (for example,
   in the case of a reply to a message which was itself a reply).  The
   "In-Reply-To:" field may be used to identify the message (or
   messages) to which the new message is a reply, while the
   "References:" field may be used to identify a "thread" of
   conversation.

   When creating a reply to a message, the "In-Reply-To:" and
   "References:" fields of the resultant message are constructed as
   follows:

   The "In-Reply-To:" field will contain the contents of the "Message-
   ID:" field of the message to which this one is a reply (the "parent
   message").  If there is more than one parent message, then the "In-
   Reply-To:" field will contain the contents of all of the parents'
   "Message-ID:" fields.  If there is no "Message-ID:" field in any of
   the parent messages, then the new message will have no "In-Reply-To:"
   field.






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   The "References:" field will contain the contents of the parent's
   "References:" field (if any) followed by the contents of the parent's
   "Message-ID:" field (if any).  If the parent message does not contain
   a "References:" field but does have an "In-Reply-To:" field
   containing a single message identifier, then the "References:" field
   will contain the contents of the parent's "In-Reply-To:" field
   followed by the contents of the parent's "Message-ID:" field (if
   any).  If the parent has none of the "References:", "In-Reply-To:",
   or "Message-ID:" fields, then the new message will have no
   "References:" field.

   Note: Some implementations parse the "References:" field to display
   the "thread of the discussion".  These implementations assume that
   each new message is a reply to a single parent and hence that they
   can walk backwards through the "References:" field to find the parent
   of each message listed there.  Therefore, trying to form a
   "References:" field for a reply that has multiple parents is
   discouraged and how to do so is not defined in this document.

   The message identifier (msg-id) itself MUST be a globally unique
   identifier for a message.  The generator of the message identifier
   MUST guarantee that the msg-id is unique.  There are several
   algorithms that can be used to accomplish this.  Since the msg-id has
   a similar syntax to angle-addr (identical except that comments and
   folding white space are not allowed), a good method is to put the
   domain name (or a domain literal IP address) of the host on which the
   message identifier was created on the right hand side of the "@", and
   put a combination of the current absolute date and time along with
   some other currently unique (perhaps sequential) identifier available
   on the system (for example, a process id number) on the left hand
   side.  Using a date on the left hand side and a domain name or domain
   literal on the right hand side makes it possible to guarantee
   uniqueness since no two hosts use the same domain name or IP address
   at the same time.  Though other algorithms will work, it is
   RECOMMENDED that the right hand side contain some domain identifier
   (either of the host itself or otherwise) such that the generator of
   the message identifier can guarantee the uniqueness of the left hand
   side within the scope of that domain.

   Semantically, the angle bracket characters are not part of the
   msg-id; the msg-id is what is contained between the two angle bracket
   characters.









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3.6.5. Informational fields

   The informational fields are all optional.  The "Keywords:" field
   contains a comma-separated list of one or more words or
   quoted-strings. The "Subject:" and "Comments:" fields are
   unstructured fields as defined in section 2.2.1, and therefore may
   contain text or folding white space.

subject         =       "Subject:" unstructured CRLF

comments        =       "Comments:" unstructured CRLF

keywords        =       "Keywords:" phrase *("," phrase) CRLF

   These three fields are intended to have only human-readable content
   with information about the message.  The "Subject:" field is the most
   common and contains a short string identifying the topic of the
   message.  When used in a reply, the field body MAY start with the
   string "Re: " (from the Latin "res", in the matter of) followed by
   the contents of the "Subject:" field body of the original message.
   If this is done, only one instance of the literal string "Re: " ought
   to be used since use of other strings or more than one instance can
   lead to undesirable consequences.  The "Comments:" field contains any
   additional comments on the text of the body of the message.  The
   "Keywords:" field contains a comma-separated list of important words
   and phrases that might be useful for the recipient.

3.6.6. Resent fields

   Resent fields SHOULD be added to any message that is reintroduced by
   a user into the transport system.  A separate set of resent fields
   SHOULD be added each time this is done.  All of the resent fields
   corresponding to a particular resending of the message SHOULD be
   together.  Each new set of resent fields is prepended to the message;
   that is, the most recent set of resent fields appear earlier in the
   message.  No other fields in the message are changed when resent
   fields are added.

   Each of the resent fields corresponds to a particular field elsewhere
   in the syntax.  For instance, the "Resent-Date:" field corresponds to
   the "Date:" field and the "Resent-To:" field corresponds to the "To:"
   field.  In each case, the syntax for the field body is identical to
   the syntax given previously for the corresponding field.

   When resent fields are used, the "Resent-From:" and "Resent-Date:"
   fields MUST be sent.  The "Resent-Message-ID:" field SHOULD be sent.
   "Resent-Sender:" SHOULD NOT be used if "Resent-Sender:" would be
   identical to "Resent-From:".



Resnick                     Standards Track                    [Page 26]

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resent-date     =       "Resent-Date:" date-time CRLF

resent-from     =       "Resent-From:" mailbox-list CRLF

resent-sender   =       "Resent-Sender:" mailbox CRLF

resent-to       =       "Resent-To:" address-list CRLF

resent-cc       =       "Resent-Cc:" address-list CRLF

resent-bcc      =       "Resent-Bcc:" (address-list / [CFWS]) CRLF

resent-msg-id   =       "Resent-Message-ID:" msg-id CRLF

   Resent fields are used to identify a message as having been
   reintroduced into the transport system by a user.  The purpose of
   using resent fields is to have the message appear to the final
   recipient as if it were sent directly by the original sender, with
   all of the original fields remaining the same.  Each set of resent
   fields correspond to a particular resending event.  That is, if a
   message is resent multiple times, each set of resent fields gives
   identifying information for each individual time.  Resent fields are
   strictly informational.  They MUST NOT be used in the normal
   processing of replies or other such automatic actions on messages.

   Note: Reintroducing a message into the transport system and using
   resent fields is a different operation from "forwarding".
   "Forwarding" has two meanings: One sense of forwarding is that a mail
   reading program can be told by a user to forward a copy of a message
   to another person, making the forwarded message the body of the new
   message.  A forwarded message in this sense does not appear to have
   come from the original sender, but is an entirely new message from
   the forwarder of the message.  On the other hand, forwarding is also
   used to mean when a mail transport program gets a message and
   forwards it on to a different destination for final delivery.  Resent
   header fields are not intended for use with either type of
   forwarding.

   The resent originator fields indicate the mailbox of the person(s) or
   system(s) that resent the message.  As with the regular originator
   fields, there are two forms: a simple "Resent-From:" form which
   contains the mailbox of the individual doing the resending, and the
   more complex form, when one individual (identified in the
   "Resent-Sender:" field) resends a message on behalf of one or more
   others (identified in the "Resent-From:" field).

   Note: When replying to a resent message, replies behave just as they
   would with any other message, using the original "From:",



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   "Reply-To:", "Message-ID:", and other fields.  The resent fields are
   only informational and MUST NOT be used in the normal processing of
   replies.

   The "Resent-Date:" indicates the date and time at which the resent
   message is dispatched by the resender of the message.  Like the
   "Date:" field, it is not the date and time that the message was
   actually transported.

   The "Resent-To:", "Resent-Cc:", and "Resent-Bcc:" fields function
   identically to the "To:", "Cc:", and "Bcc:" fields respectively,
   except that they indicate the recipients of the resent message, not
   the recipients of the original message.

   The "Resent-Message-ID:" field provides a unique identifier for the
   resent message.

3.6.7. Trace fields

   The trace fields are a group of header fields consisting of an
   optional "Return-Path:" field, and one or more "Received:" fields.
   The "Return-Path:" header field contains a pair of angle brackets
   that enclose an optional addr-spec.  The "Received:" field contains a
   (possibly empty) list of name/value pairs followed by a semicolon and
   a date-time specification.  The first item of the name/value pair is
   defined by item-name, and the second item is either an addr-spec, an
   atom, a domain, or a msg-id.  Further restrictions may be applied to
   the syntax of the trace fields by standards that provide for their
   use, such as [RFC2821].

trace           =       [return]
                        1*received

return          =       "Return-Path:" path CRLF

path            =       ([CFWS] "<" ([CFWS] / addr-spec) ">" [CFWS]) /
                        obs-path

received        =       "Received:" name-val-list ";" date-time CRLF

name-val-list   =       [CFWS] [name-val-pair *(CFWS name-val-pair)]

name-val-pair   =       item-name CFWS item-value

item-name       =       ALPHA *(["-"] (ALPHA / DIGIT))

item-value      =       1*angle-addr / addr-spec /
                         atom / domain / msg-id



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   A full discussion of the Internet mail use of trace fields is
   contained in [RFC2821].  For the purposes of this standard, the trace
   fields are strictly informational, and any formal interpretation of
   them is outside of the scope of this document.

3.6.8. Optional fields

   Fields may appear in messages that are otherwise unspecified in this
   standard.  They MUST conform to the syntax of an optional-field.
   This is a field name, made up of the printable US-ASCII characters
   except SP and colon, followed by a colon, followed by any text which
   conforms to unstructured.

   The field names of any optional-field MUST NOT be identical to any
   field name specified elsewhere in this standard.

optional-field  =       field-name ":" unstructured CRLF

field-name      =       1*ftext

ftext           =       %d33-57 /               ; Any character except
                        %d59-126                ;  controls, SP, and
                                                ;  ":".

   For the purposes of this standard, any optional field is
   uninterpreted.

4. Obsolete Syntax

   Earlier versions of this standard allowed for different (usually more
   liberal) syntax than is allowed in this version.  Also, there have
   been syntactic elements used in messages on the Internet whose
   interpretation have never been documented.  Though some of these
   syntactic forms MUST NOT be generated according to the grammar in
   section 3, they MUST be accepted and parsed by a conformant receiver.
   This section documents many of these syntactic elements.  Taking the
   grammar in section 3 and adding the definitions presented in this
   section will result in the grammar to use for interpretation of
   messages.

   Note: This section identifies syntactic forms that any implementation
   MUST reasonably interpret.  However, there are certainly Internet
   messages which do not conform to even the additional syntax given in
   this section.  The fact that a particular form does not appear in any
   section of this document is not justification for computer programs
   to crash or for malformed data to be irretrievably lost by any
   implementation.  To repeat an example, though this document requires
   lines in messages to be no longer than 998 characters, silently



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   discarding the 999th and subsequent characters in a line without
   warning would still be bad behavior for an implementation.  It is up
   to the implementation to deal with messages robustly.

   One important difference between the obsolete (interpreting) and the
   current (generating) syntax is that in structured header field bodies
   (i.e., between the colon and the CRLF of any structured header
   field), white space characters, including folding white space, and
   comments can be freely inserted between any syntactic tokens.  This
   allows many complex forms that have proven difficult for some
   implementations to parse.

   Another key difference between the obsolete and the current syntax is
   that the rule in section 3.2.3 regarding lines composed entirely of
   white space in comments and folding white space does not apply.  See
   the discussion of folding white space in section 4.2 below.

   Finally, certain characters that were formerly allowed in messages
   appear in this section.  The NUL character (ASCII value 0) was once
   allowed, but is no longer for compatibility reasons.  CR and LF were
   allowed to appear in messages other than as CRLF; this use is also
   shown here.

   Other differences in syntax and semantics are noted in the following
   sections.

4.1. Miscellaneous obsolete tokens

   These syntactic elements are used elsewhere in the obsolete syntax or
   in the main syntax.  The obs-char and obs-qp elements each add ASCII
   value 0. Bare CR and bare LF are added to obs-text and obs-utext.
   The period character is added to obs-phrase. The obs-phrase-list
   provides for "empty" elements in a comma-separated list of phrases.

   Note: The "period" (or "full stop") character (".") in obs-phrase is
   not a form that was allowed in earlier versions of this or any other
   standard.  Period (nor any other character from specials) was not
   allowed in phrase because it introduced a parsing difficulty
   distinguishing between phrases and portions of an addr-spec (see
   section 4.4).  It appears here because the period character is
   currently used in many messages in the display-name portion of
   addresses, especially for initials in names, and therefore must be
   interpreted properly.  In the future, period may appear in the
   regular syntax of phrase.

obs-qp          =       "\" (%d0-127)

obs-text        =       *LF *CR *(obs-char *LF *CR)



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obs-char        =       %d0-9 / %d11 /          ; %d0-127 except CR and
                        %d12 / %d14-127         ;  LF

obs-utext       =       obs-text

obs-phrase      =       word *(word / "." / CFWS)

obs-phrase-list =       phrase / 1*([phrase] [CFWS] "," [CFWS]) [phrase]

   Bare CR and bare LF appear in messages with two different meanings.
   In many cases, bare CR or bare LF are used improperly instead of CRLF
   to indicate line separators.  In other cases, bare CR and bare LF are
   used simply as ASCII control characters with their traditional ASCII
   meanings.

4.2. Obsolete folding white space

   In the obsolete syntax, any amount of folding white space MAY be
   inserted where the obs-FWS rule is allowed.  This creates the
   possibility of having two consecutive "folds" in a line, and
   therefore the possibility that a line which makes up a folded header
   field could be composed entirely of white space.

   obs-FWS         =       1*WSP *(CRLF 1*WSP)

4.3. Obsolete Date and Time

   The syntax for the obsolete date format allows a 2 digit year in the
   date field and allows for a list of alphabetic time zone
   specifications that were used in earlier versions of this standard.
   It also permits comments and folding white space between many of the
   tokens.

obs-day-of-week =       [CFWS] day-name [CFWS]

obs-year        =       [CFWS] 2*DIGIT [CFWS]

obs-month       =       CFWS month-name CFWS

obs-day         =       [CFWS] 1*2DIGIT [CFWS]

obs-hour        =       [CFWS] 2DIGIT [CFWS]

obs-minute      =       [CFWS] 2DIGIT [CFWS]

obs-second      =       [CFWS] 2DIGIT [CFWS]

obs-zone        =       "UT" / "GMT" /          ; Universal Time



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                                                ; North American UT
                                                ; offsets
                        "EST" / "EDT" /         ; Eastern:  - 5/ - 4
                        "CST" / "CDT" /         ; Central:  - 6/ - 5
                        "MST" / "MDT" /         ; Mountain: - 7/ - 6
                        "PST" / "PDT" /         ; Pacific:  - 8/ - 7

                        %d65-73 /               ; Military zones - "A"
                        %d75-90 /               ; through "I" and "K"
                        %d97-105 /              ; through "Z", both
                        %d107-122               ; upper and lower case

   Where a two or three digit year occurs in a date, the year is to be
   interpreted as follows: If a two digit year is encountered whose
   value is between 00 and 49, the year is interpreted by adding 2000,
   ending up with a value between 2000 and 2049.  If a two digit year is
   encountered with a value between 50 and 99, or any three digit year
   is encountered, the year is interpreted by adding 1900.

   In the obsolete time zone, "UT" and "GMT" are indications of
   "Universal Time" and "Greenwich Mean Time" respectively and are both
   semantically identical to "+0000".

   The remaining three character zones are the US time zones.  The first
   letter, "E", "C", "M", or "P" stands for "Eastern", "Central",
   "Mountain" and "Pacific".  The second letter is either "S" for
   "Standard" time, or "D" for "Daylight" (or summer) time.  Their
   interpretations are as follows:

   EDT is semantically equivalent to -0400
   EST is semantically equivalent to -0500
   CDT is semantically equivalent to -0500
   CST is semantically equivalent to -0600
   MDT is semantically equivalent to -0600
   MST is semantically equivalent to -0700
   PDT is semantically equivalent to -0700
   PST is semantically equivalent to -0800

   The 1 character military time zones were defined in a non-standard
   way in [RFC822] and are therefore unpredictable in their meaning.
   The original definitions of the military zones "A" through "I" are
   equivalent to "+0100" through "+0900" respectively; "K", "L", and "M"
   are equivalent to  "+1000", "+1100", and "+1200" respectively; "N"
   through "Y" are equivalent to "-0100" through "-1200" respectively;
   and "Z" is equivalent to "+0000".  However, because of the error in
   [RFC822], they SHOULD all be considered equivalent to "-0000" unless
   there is out-of-band information confirming their meaning.




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   Other multi-character (usually between 3 and 5) alphabetic time zones
   have been used in Internet messages.  Any such time zone whose
   meaning is not known SHOULD be considered equivalent to "-0000"
   unless there is out-of-band information confirming their meaning.

4.4. Obsolete Addressing

   There are three primary differences in addressing.  First, mailbox
   addresses were allowed to have a route portion before the addr-spec
   when enclosed in "<" and ">".  The route is simply a comma-separated
   list of domain names, each preceded by "@", and the list terminated
   by a colon.  Second, CFWS were allowed between the period-separated
   elements of local-part and domain (i.e., dot-atom was not used).  In
   addition, local-part is allowed to contain quoted-string in addition
   to just atom.  Finally, mailbox-list and address-list were allowed to
   have "null" members.  That is, there could be two or more commas in
   such a list with nothing in between them.

obs-angle-addr  =       [CFWS] "<" [obs-route] addr-spec ">" [CFWS]

obs-route       =       [CFWS] obs-domain-list ":" [CFWS]

obs-domain-list =       "@" domain *(*(CFWS / "," ) [CFWS] "@" domain)

obs-local-part  =       word *("." word)

obs-domain      =       atom *("." atom)

obs-mbox-list   =       1*([mailbox] [CFWS] "," [CFWS]) [mailbox]

obs-addr-list   =       1*([address] [CFWS] "," [CFWS]) [address]

   When interpreting addresses, the route portion SHOULD be ignored.

4.5. Obsolete header fields

   Syntactically, the primary difference in the obsolete field syntax is
   that it allows multiple occurrences of any of the fields and they may
   occur in any order.  Also, any amount of white space is allowed
   before the ":" at the end of the field name.

obs-fields      =       *(obs-return /
                        obs-received /
                        obs-orig-date /
                        obs-from /
                        obs-sender /
                        obs-reply-to /
                        obs-to /



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                        obs-cc /
                        obs-bcc /
                        obs-message-id /
                        obs-in-reply-to /
                        obs-references /
                        obs-subject /
                        obs-comments /
                        obs-keywords /
                        obs-resent-date /
                        obs-resent-from /
                        obs-resent-send /
                        obs-resent-rply /
                        obs-resent-to /
                        obs-resent-cc /
                        obs-resent-bcc /
                        obs-resent-mid /
                        obs-optional)

   Except for destination address fields (described in section 4.5.3),
   the interpretation of multiple occurrences of fields is unspecified.
   Also, the interpretation of trace fields and resent fields which do
   not occur in blocks prepended to the message is unspecified as well.
   Unless otherwise noted in the following sections, interpretation of
   other fields is identical to the interpretation of their non-obsolete
   counterparts in section 3.

4.5.1. Obsolete origination date field

obs-orig-date   =       "Date" *WSP ":" date-time CRLF

4.5.2. Obsolete originator fields

obs-from        =       "From" *WSP ":" mailbox-list CRLF

obs-sender      =       "Sender" *WSP ":" mailbox CRLF

obs-reply-to    =       "Reply-To" *WSP ":" mailbox-list CRLF

4.5.3. Obsolete destination address fields

obs-to          =       "To" *WSP ":" address-list CRLF

obs-cc          =       "Cc" *WSP ":" address-list CRLF

obs-bcc         =       "Bcc" *WSP ":" (address-list / [CFWS]) CRLF






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   When multiple occurrences of destination address fields occur in a
   message, they SHOULD be treated as if the address-list in the first
   occurrence of the field is combined with the address lists of the
   subsequent occurrences by adding a comma and concatenating.

4.5.4. Obsolete identification fields

   The obsolete "In-Reply-To:" and "References:" fields differ from the
   current syntax in that they allow phrase (words or quoted strings) to
   appear.  The obsolete forms of the left and right sides of msg-id
   allow interspersed CFWS, making them syntactically identical to
   local-part and domain respectively.

obs-message-id  =       "Message-ID" *WSP ":" msg-id CRLF

obs-in-reply-to =       "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF

obs-references  =       "References" *WSP ":" *(phrase / msg-id) CRLF

obs-id-left     =       local-part

obs-id-right    =       domain

   For purposes of interpretation, the phrases in the "In-Reply-To:" and
   "References:" fields are ignored.

   Semantically, none of the optional CFWS surrounding the local-part
   and the domain are part of the obs-id-left and obs-id-right
   respectively.

4.5.5. Obsolete informational fields

obs-subject     =       "Subject" *WSP ":" unstructured CRLF

obs-comments    =       "Comments" *WSP ":" unstructured CRLF

obs-keywords    =       "Keywords" *WSP ":" obs-phrase-list CRLF

4.5.6. Obsolete resent fields

   The obsolete syntax adds a "Resent-Reply-To:" field, which consists
   of the field name, the optional comments and folding white space, the
   colon, and a comma separated list of addresses.

obs-resent-from =       "Resent-From" *WSP ":" mailbox-list CRLF

obs-resent-send =       "Resent-Sender" *WSP ":" mailbox CRLF




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obs-resent-date =       "Resent-Date" *WSP ":" date-time CRLF

obs-resent-to   =       "Resent-To" *WSP ":" address-list CRLF

obs-resent-cc   =       "Resent-Cc" *WSP ":" address-list CRLF

obs-resent-bcc  =       "Resent-Bcc" *WSP ":"
                         (address-list / [CFWS]) CRLF

obs-resent-mid  =       "Resent-Message-ID" *WSP ":" msg-id CRLF

obs-resent-rply =       "Resent-Reply-To" *WSP ":" address-list CRLF

   As with other resent fields, the "Resent-Reply-To:" field is to be
   treated as trace information only.

4.5.7. Obsolete trace fields

   The obs-return and obs-received are again given here as template
   definitions, just as return and received are in section 3.  Their
   full syntax is given in [RFC2821].

obs-return      =       "Return-Path" *WSP ":" path CRLF

obs-received    =       "Received" *WSP ":" name-val-list CRLF

obs-path        =       obs-angle-addr

4.5.8. Obsolete optional fields

obs-optional    =       field-name *WSP ":" unstructured CRLF

5. Security Considerations

   Care needs to be taken when displaying messages on a terminal or
   terminal emulator.  Powerful terminals may act on escape sequences
   and other combinations of ASCII control characters with a variety of
   consequences.  They can remap the keyboard or permit other
   modifications to the terminal which could lead to denial of service
   or even damaged data.  They can trigger (sometimes programmable)
   answerback messages which can allow a message to cause commands to be
   issued on the recipient's behalf.  They can also effect the operation
   of terminal attached devices such as printers.  Message viewers may
   wish to strip potentially dangerous terminal escape sequences from
   the message prior to display.  However, other escape sequences appear
   in messages for useful purposes (cf. [RFC2045, RFC2046, RFC2047,
   RFC2048, RFC2049, ISO2022]) and therefore should not be stripped
   indiscriminately.



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   Transmission of non-text objects in messages raises additional
   security issues.  These issues are discussed in [RFC2045, RFC2046,
   RFC2047, RFC2048, RFC2049].

   Many implementations use the "Bcc:" (blind carbon copy) field
   described in section 3.6.3 to facilitate sending messages to
   recipients without revealing the addresses of one or more of the
   addressees to the other recipients.  Mishandling this use of "Bcc:"
   has implications for confidential information that might be revealed,
   which could eventually lead to security problems through knowledge of
   even the existence of a particular mail address.  For example, if
   using the first method described in section 3.6.3, where the "Bcc:"
   line is removed from the message, blind recipients have no explicit
   indication that they have been sent a blind copy, except insofar as
   their address does not appear in the message header.  Because of
   this, one of the blind addressees could potentially send a reply to
   all of the shown recipients and accidentally reveal that the message
   went to the blind recipient.  When the second method from section
   3.6.3 is used, the blind recipient's address appears in the "Bcc:"
   field of a separate copy of the message. If the "Bcc:" field sent
   contains all of the blind addressees, all of the "Bcc:" recipients
   will be seen by each "Bcc:" recipient.  Even if a separate message is
   sent to each "Bcc:" recipient with only the individual's address,
   implementations still need to be careful to process replies to the
   message as per section 3.6.3 so as not to accidentally reveal the
   blind recipient to other recipients.

6. Bibliography

   [ASCII]    American National Standards Institute (ANSI), Coded
              Character Set - 7-Bit American National Standard Code for
              Information Interchange, ANSI X3.4, 1986.

   [ISO2022] International Organization for Standardization (ISO),
              Information processing - ISO 7-bit and 8-bit coded
              character sets - Code extension techniques, Third edition
              - 1986-05-01, ISO 2022, 1986.

   [RFC822]   Crocker, D., "Standard for the Format of ARPA Internet
              Text Messages", RFC 822, August 1982.

   [RFC2045]  Freed, N. and  N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.



Resnick                     Standards Track                    [Page 37]

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   [RFC2047]  Moore, K., "Multipurpose Internet Mail Extensions (MIME)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2048]  Freed, N., Klensin, J. and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Format of
              Internet Message Bodies", RFC 2048, November 1996.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2234]  Crocker, D., Editor, and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", RFC 2234, November 1997.

   [RFC2821]  Klensin, J., Editor, "Simple Mail Transfer Protocol", RFC
              2821, March 2001.

   [STD3]     Braden, R., "Host Requirements", STD 3, RFC 1122 and RFC
              1123, October 1989.

   [STD12]    Mills, D., "Network Time Protocol", STD 12, RFC 1119,
              September 1989.

   [STD13]    Mockapetris, P., "Domain Name System", STD 13, RFC 1034
              and RFC 1035,  November 1987.

   [STD14]    Partridge, C., "Mail Routing and the Domain System", STD
              14, RFC 974, January 1986.

7. Editor's Address

   Peter W. Resnick
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA 92121-1714
   USA

   Phone: +1 858 651 4478
   Fax:   +1 858 651 1102
   EMail: presnick@qualcomm.com







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8. Acknowledgements

   Many people contributed to this document.  They included folks who
   participated in the Detailed Revision and Update of Messaging
   Standards (DRUMS) Working Group of the Internet Engineering Task
   Force (IETF), the chair of DRUMS, the Area Directors of the IETF, and
   people who simply sent their comments in via e-mail.  The editor is
   deeply indebted to them all and thanks them sincerely.  The below
   list includes everyone who sent e-mail concerning this document.
   Hopefully, everyone who contributed is named here:

   Matti Aarnio              Barry Finkel           Larry Masinter
   Tanaka Akira              Erik Forsberg          Denis McKeon
   Russ Allbery              Chuck Foster           William P McQuillan
   Eric Allman               Paul Fox               Alexey Melnikov
   Harald Tveit Alvestrand   Klaus M. Frank         Perry E. Metzger
   Ran Atkinson              Ned Freed              Steven Miller
   Jos Backus                Jochen Friedrich       Keith Moore
   Bruce Balden              Randall C. Gellens     John Gardiner Myers
   Dave Barr                 Sukvinder Singh Gill   Chris Newman
   Alan Barrett              Tim Goodwin            John W. Noerenberg
   John Beck                 Philip Guenther        Eric Norman
   J. Robert von Behren      Tony Hansen            Mike O'Dell
   Jos den Bekker            John Hawkinson         Larry Osterman
   D. J. Bernstein           Philip Hazel           Paul Overell
   James Berriman            Kai Henningsen         Jacob Palme
   Norbert Bollow            Robert Herriot         Michael A. Patton
   Raj Bose                  Paul Hethmon           Uzi Paz
   Antony Bowesman           Jim Hill               Michael A. Quinlan
   Scott Bradner             Paul E. Hoffman        Eric S. Raymond
   Randy Bush                Steve Hole             Sam Roberts
   Tom Byrer                 Kari Hurtta            Hugh Sasse
   Bruce Campbell            Marco S. Hyman         Bart Schaefer
   Larry Campbell            Ofer Inbar             Tom Scola
   W. J. Carpenter           Olle Jarnefors         Wolfgang Segmuller
   Michael Chapman           Kevin Johnson          Nick Shelness
   Richard Clayton           Sudish Joseph          John Stanley
   Maurizio Codogno          Maynard Kang           Einar Stefferud
   Jim Conklin               Prabhat Keni           Jeff Stephenson
   R. Kelley Cook            John C. Klensin        Bernard Stern
   Steve Coya                Graham Klyne           Peter Sylvester
   Mark Crispin              Brad Knowles           Mark Symons
   Dave Crocker              Shuhei Kobayashi       Eric Thomas
   Matt Curtin               Peter Koch             Lee Thompson
   Michael D'Errico          Dan Kohn               Karel De Vriendt
   Cyrus Daboo               Christian Kuhtz        Matthew Wall
   Jutta Degener             Anand Kumria           Rolf Weber
   Mark Delany               Steen Larsen           Brent B. Welch



Resnick                     Standards Track                    [Page 39]

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   Steve Dorner              Eliot Lear             Dan Wing
   Harold A. Driscoll        Barry Leiba            Jack De Winter
   Michael Elkins            Jay Levitt             Gregory J. Woodhouse
   Robert Elz                Lars-Johan Liman       Greg A. Woods
   Johnny Eriksson           Charles Lindsey        Kazu Yamamoto
   Erik E. Fair              Pete Loshin            Alain Zahm
   Roger Fajman              Simon Lyall            Jamie Zawinski
   Patrik Faltstrom          Bill Manning           Timothy S. Zurcher
   Claus Andre Farber        John Martin










































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Appendix A. Example messages

   This section presents a selection of messages.  These are intended to
   assist in the implementation of this standard, but should not be
   taken as normative; that is to say, although the examples in this
   section were carefully reviewed, if there happens to be a conflict
   between these examples and the syntax described in sections 3 and 4
   of this document, the syntax in those sections is to be taken as
   correct.

   Messages are delimited in this section between lines of "----".  The
   "----" lines are not part of the message itself.

A.1. Addressing examples

   The following are examples of messages that might be sent between two
   individuals.

A.1.1. A message from one person to another with simple addressing

   This could be called a canonical message.  It has a single author,
   John Doe, a single recipient, Mary Smith, a subject, the date, a
   message identifier, and a textual message in the body.

----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----

















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   If John's secretary Michael actually sent the message, though John
   was the author and replies to this message should go back to him, the
   sender field would be used:

----
From: John Doe <jdoe@machine.example>
Sender: Michael Jones <mjones@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----

A.1.2. Different types of mailboxes

   This message includes multiple addresses in the destination fields
   and also uses several different forms of addresses.

----
From: "Joe Q. Public" <john.q.public@example.com>
To: Mary Smith <mary@x.test>, jdoe@example.org, Who? <one@y.test>
Cc: <boss@nil.test>, "Giant; \"Big\" Box" <sysservices@example.net>
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <5678.21-Nov-1997@example.com>

Hi everyone.
----

   Note that the display names for Joe Q. Public and Giant; "Big" Box
   needed to be enclosed in double-quotes because the former contains
   the period and the latter contains both semicolon and double-quote
   characters (the double-quote characters appearing as quoted-pair
   construct).  Conversely, the display name for Who? could appear
   without them because the question mark is legal in an atom.  Notice
   also that jdoe@example.org and boss@nil.test have no display names
   associated with them at all, and jdoe@example.org uses the simpler
   address form without the angle brackets.











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A.1.3. Group addresses

----
From: Pete <pete@silly.example>
To: A Group:Chris Jones <c@a.test>,joe@where.test,John <jdoe@one.test>;
Cc: Undisclosed recipients:;
Date: Thu, 13 Feb 1969 23:32:54 -0330
Message-ID: <testabcd.1234@silly.example>

Testing.
----

   In this message, the "To:" field has a single group recipient named A
   Group which contains 3 addresses, and a "Cc:" field with an empty
   group recipient named Undisclosed recipients.

A.2. Reply messages

   The following is a series of three messages that make up a
   conversation thread between John and Mary.  John firsts sends a
   message to Mary, Mary then replies to John's message, and then John
   replies to Mary's reply message.

   Note especially the "Message-ID:", "References:", and "In-Reply-To:"
   fields in each message.

----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----















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   When sending replies, the Subject field is often retained, though
   prepended with "Re: " as described in section 3.6.5.

----
From: Mary Smith <mary@example.net>
To: John Doe <jdoe@machine.example>
Reply-To: "Mary Smith: Personal Account" <smith@home.example>
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 10:01:10 -0600
Message-ID: <3456@example.net>
In-Reply-To: <1234@local.machine.example>
References: <1234@local.machine.example>

This is a reply to your hello.
----

   Note the "Reply-To:" field in the above message.  When John replies
   to Mary's message above, the reply should go to the address in the
   "Reply-To:" field instead of the address in the "From:" field.

----
To: "Mary Smith: Personal Account" <smith@home.example>
From: John Doe <jdoe@machine.example>
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 11:00:00 -0600
Message-ID: <abcd.1234@local.machine.tld>
In-Reply-To: <3456@example.net>
References: <1234@local.machine.example> <3456@example.net>

This is a reply to your reply.
----

A.3. Resent messages

   Start with the message that has been used as an example several
   times:

----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----




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   Say that Mary, upon receiving this message, wishes to send a copy of
   the message to Jane such that (a) the message would appear to have
   come straight from John; (b) if Jane replies to the message, the
   reply should go back to John; and (c) all of the original
   information, like the date the message was originally sent to Mary,
   the message identifier, and the original addressee, is preserved.  In
   this case, resent fields are prepended to the message:

----
Resent-From: Mary Smith <mary@example.net>
Resent-To: Jane Brown <j-brown@other.example>
Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800
Resent-Message-ID: <78910@example.net>
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----

   If Jane, in turn, wished to resend this message to another person,
   she would prepend her own set of resent header fields to the above
   and send that.

























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A.4. Messages with trace fields

   As messages are sent through the transport system as described in
   [RFC2821], trace fields are prepended to the message.  The following
   is an example of what those trace fields might look like.  Note that
   there is some folding white space in the first one since these lines
   can be long.

----
Received: from x.y.test
   by example.net
   via TCP
   with ESMTP
   id ABC12345
   for <mary@example.net>;  21 Nov 1997 10:05:43 -0600
Received: from machine.example by x.y.test; 21 Nov 1997 10:01:22 -0600
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----


























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A.5. White space, comments, and other oddities

   White space, including folding white space, and comments can be
   inserted between many of the tokens of fields.  Taking the example
   from A.1.3, white space and comments can be inserted into all of the
   fields.

----
From: Pete(A wonderful \) chap) <pete(his account)@silly.test(his host)>
To:A Group(Some people)
     :Chris Jones <c@(Chris's host.)public.example>,
         joe@example.org,
  John <jdoe@one.test> (my dear friend); (the end of the group)
Cc:(Empty list)(start)Undisclosed recipients  :(nobody(that I know))  ;
Date: Thu,
      13
        Feb
          1969
      23:32
               -0330 (Newfoundland Time)
Message-ID:              <testabcd.1234@silly.test>

Testing.
----

   The above example is aesthetically displeasing, but perfectly legal.
   Note particularly (1) the comments in the "From:" field (including
   one that has a ")" character appearing as part of a quoted-pair); (2)
   the white space absent after the ":" in the "To:" field as well as
   the comment and folding white space after the group name, the special
   character (".") in the comment in Chris Jones's address, and the
   folding white space before and after "joe@example.org,"; (3) the
   multiple and nested comments in the "Cc:" field as well as the
   comment immediately following the ":" after "Cc"; (4) the folding
   white space (but no comments except at the end) and the missing
   seconds in the time of the date field; and (5) the white space before
   (but not within) the identifier in the "Message-ID:" field.

A.6. Obsoleted forms

   The following are examples of obsolete (that is, the "MUST NOT
   generate") syntactic elements described in section 4 of this
   document.








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A.6.1. Obsolete addressing

   Note in the below example the lack of quotes around Joe Q. Public,
   the route that appears in the address for Mary Smith, the two commas
   that appear in the "To:" field, and the spaces that appear around the
   "." in the jdoe address.

----
From: Joe Q. Public <john.q.public@example.com>
To: Mary Smith <@machine.tld:mary@example.net>, , jdoe@test   . example
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <5678.21-Nov-1997@example.com>

Hi everyone.
----

A.6.2. Obsolete dates

   The following message uses an obsolete date format, including a non-
   numeric time zone and a two digit year.  Note that although the
   day-of-week is missing, that is not specific to the obsolete syntax;
   it is optional in the current syntax as well.

----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: 21 Nov 97 09:55:06 GMT
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----

A.6.3. Obsolete white space and comments

   White space and comments can appear between many more elements than
   in the current syntax.  Also, folding lines that are made up entirely
   of white space are legal.












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----
From  : John Doe <jdoe@machine(comment).  example>
To    : Mary Smith
__
          <mary@example.net>
Subject     : Saying Hello
Date  : Fri, 21 Nov 1997 09(comment):   55  :  06 -0600
Message-ID  : <1234   @   local(blah)  .machine .example>

This is a message just to say hello.
So, "Hello".
----

   Note especially the second line of the "To:" field.  It starts with
   two space characters.  (Note that "__" represent blank spaces.)
   Therefore, it is considered part of the folding as described in
   section 4.2.  Also, the comments and white space throughout
   addresses, dates, and message identifiers are all part of the
   obsolete syntax.

Appendix B. Differences from earlier standards

   This appendix contains a list of changes that have been made in the
   Internet Message Format from earlier standards, specifically [RFC822]
   and [STD3].  Items marked with an asterisk (*) below are items which
   appear in section 4 of this document and therefore can no longer be
   generated.

   1. Period allowed in obsolete form of phrase.
   2. ABNF moved out of document to [RFC2234].
   3. Four or more digits allowed for year.
   4. Header field ordering (and lack thereof) made explicit.
   5. Encrypted header field removed.
   6. Received syntax loosened to allow any token/value pair.
   7. Specifically allow and give meaning to "-0000" time zone.
   8. Folding white space is not allowed between every token.
   9. Requirement for destinations removed.
   10. Forwarding and resending redefined.
   11. Extension header fields no longer specifically called out.
   12. ASCII 0 (null) removed.*
   13. Folding continuation lines cannot contain only white space.*
   14. Free insertion of comments not allowed in date.*
   15. Non-numeric time zones not allowed.*
   16. Two digit years not allowed.*
   17. Three digit years interpreted, but not allowed for generation.
   18. Routes in addresses not allowed.*
   19. CFWS within local-parts and domains not allowed.*
   20. Empty members of address lists not allowed.*



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   21. Folding white space between field name and colon not allowed.*
   22. Comments between field name and colon not allowed.
   23. Tightened syntax of in-reply-to and references.*
   24. CFWS within msg-id not allowed.*
   25. Tightened semantics of resent fields as informational only.
   26. Resent-Reply-To not allowed.*
   27. No multiple occurrences of fields (except resent and received).*
   28. Free CR and LF not allowed.*
   29. Routes in return path not allowed.*
   30. Line length limits specified.
   31. Bcc more clearly specified.

Appendix C. Notices

   Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.






















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Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















Resnick                     Standards Track                    [Page 51]